This invention generally relates to a threaded joint for steel pipes for use in connecting oil well pipes to each other. More particularly, this invention relates to a threaded joint for steel pipes which has a solid lubricant coating having excellent galling resistance, gas tightness, and rust preventing properties and which does not require the application of a compound grease containing a heavy metal powder, which application was conventionally carried out before each time fastening was performed in order to prevent the joint from galling, and to a process for surface treatment capable of forming such a solid lubricant coating.
Oil well pipes which are steel pipes used in the drilling of oil wells are connected with each other by a threaded joint for steel pipes. The threaded joint is comprised of a pin having a male thread and a box having a female thread.
As schematically shown in FIG. 1, a male thread 3A is normally formed on the outer surface at both ends of a steel pipe A to form a pin 1, and a female thread 3B is formed on both sides of the inner surface of a separate joint member in the form of a sleeve-shaped coupling B to form a box 2. As shown in FIG. 1, the steel pipe A is normally shipped in a state in which a coupling B is previously connected to one end.
A threaded joint for steel pipes is subjected to compound pressures due to axial tensile forces caused by the weight of the steel pipe and the coupling and internal and external pressures underground, and it is also subjected to heat underground. Therefore, a threaded joint is required to maintain gas tightness (sealability) without being damaged even under such conditions. In addition, during the process of lowering oil well pipes, it is often the case that a joint which has once been fastened is loosened (unfastened) and then refastened. Therefore, according to API (American Petroleum Institute), it is desired that there be no occurrence of severe seizing called galling and that gas tightness be maintained even if fastening (make-up) and loosening (break-out) are carried out ten times for joints for tubing and three times for joints for casing.
In recent years, in order to improve gas tightness, special threaded joints which are capable of forming a metal-to-metal seal have come to be generally used. In this type of threaded joint, each of a pin and a box has an unthreaded metal contact portion in addition to a threaded portion having a male or female thread, and both the threaded portion and the unthreaded metal contact portion form a contact surface between the pin and box. The unthreaded metal contact portions of the pin and the box come into intimate contact with each other to form a metal-to-metal seal portion and contribute to an increase in gas tightness.
In such a threaded joint capable of forming a metal-to-metal seal, a lubricating grease with high lubricity called a compound grease has been used. This grease, which is a kind of liquid lubricant, is applied to the contact surface of at least one of the pin and the box prior to fastening. However, this grease contains a large amount of harmful heavy metal powders. When the grease which is squeezed out to the periphery during fastening is cleaned with a cleaning agent, the compound grease and the used cleaning agent flow out into the ocean or the soil and cause environmental pollution, and this has come to be considered a problem. In addition, there was the problem that the application of grease and cleaning which were repeated before each fastening lowered the working efficiency in the field.
As threaded joints for steel pipes which do not need the application of a compound grease, JP 08-103724A, JP 08-233163A, JP 08-233164A, and JP 09-72467A disclose threaded joints in which a solid lubricant coating comprising a resin as a binder and molybdenum disulfide or tungsten disulfide as a solid lubricant is applied to a threaded portion and an unthreaded metal contact portion (namely, to the contact surface) of at least one of a pin and a box.
In these Japanese patent publications, in order to increase the adhesion between the solid lubricant coating and the substrate steel, it is disclosed to form, as an undercoating layer for the solid lubricant coating, a manganese phosphate chemical conversion coating layer or a combination of a nitride layer and a manganese phosphate chemical conversion coating layer, or to provide the contact surface with surface roughness having an Rmax of 5-40 xcexcm. JP 08-103724A discloses that a solid lubricant coating is formed by performing baking of an applied coating with heating for 20-30 minutes in the temperature range of 150-300xc2x0 C.
It might be expected that the use of a threaded joint in which the contact surface of a pin and a box has a solid lubricant coating formed by surface treatment to provide lubricity thereto would make it possible to dispense with the application of a compound grease and thus avoid the aforementioned problems regarding the environment and working efficiency.
However, with a conventional solid lubricant coating, it is not possible to attain a high anti-galling effect such as can be obtained by application of a compound grease, and a seizing flaw called galling occurs after fastening and loosening are repeated several times. Thus, the effect of a conventional solid lubricant coating for preventing galling was insufficient.
The decrease in galling resistance and gas tightness of a threaded joint was significant, particularly when the storage period of the threaded joint from its shipping out of the factory (i.e., from the formation of a solid lubricant coating) to its actual use on a rig site to fasten it was long (it is sometimes as long as one or two years).
Furthermore, recently, a heat-resistant threaded joint for steel pipes has been desired for use in high-temperature oil wells in which the temperature reaches 250-300xc2x0 C., which is higher than the temperature in conventional oil wells, or in steam-injection oil wells into which steam at a high temperature close to the critical temperature (e.g., around 350xc2x0 C.) is injected in order to improve oil recovery. Therefore, it has been required for a threaded joint to guarantee galling resistance and gas tightness when a joint which has been fastened is subjected to a heating test at a temperature of around 350xc2x0 C. and then subjected to loosening and re-fastening. With the above-described conventional solid lubricant coating, it was difficult to assure these properties required for a heat-resistant threaded joint.
It is an object of this invention to provide a process for the heat treatment of a threaded joint for steel pipes, which can form a solid lubricant coating having excellent galling resistance which can effectively suppress the occurrence of galling upon repeated fastening and loosening even with a heat-resistant threaded joint for steel pipes.
It is another object of this invention to provide a threaded joint for steel pipes which can alleviate a decrease in galling resistance and gas tightness without using a compound grease when it is stored for a prolonged period from the formation of a solid lubricant coating to its use on site.
According to one aspect, the present invention is a process for the surface treatment of a threaded joint for steel pipes comprising a pin and a box each having a contact surface including a threaded portion and an unthreaded metal contact portion, characterized in that the process comprises the steps:
applying a coating fluid containing a resin binder and a lubricating powder in a solvent to the contact surface of at least one of the pin and the box, and
drying the applied coating by multistage heating which includes at least first stage heating in the temperature range of from 70xc2x0 C. to 150xc2x0 C. and second stage heating in the range of from higher than 150xc2x0 C. to 380xc2x0 C. to form a solid lubricant coating on the contact surface.
The process may further include, prior to the coating application step, a step of heating the contact surface to be coated to a temperature of from 50xc2x0 C. to 200xc2x0 C.
The solid lubricant coating formed according to the process of the present invention can possess a hardness of 70-140 on the Rockwell M scale and an adhesive strength of at least 500 N/m as determined by the SAICAS (Surface And Interfacial Cutting Analysis System) method.
It has been found that a cause of insufficient galling resistance of a conventional solid lubricant coating formed on the contact surface of a threaded joint for steel pipes is insufficient hardness of the coating, which is caused by insufficient drying of the coating.
A solid lubricant coating for a threaded joint is generally formed by applying a coating fluid containing a resin and a lubricating powder (e.g., molybdenum disulfide powder) in a volatile solvent to the contact surface of the threaded joint, followed by heating to dry (or bake) the applied coating. In the case where the applied coating is dried by heating at a temperature of 150-300xc2x0 C. as employed in the prior art, even if the heating is carried out for a prolonged period, it is not possible to completely evaporate the solvent, and a minute amount of the solvent and moisture is confined in the dried coating and leads to the formation of internal defects, which prevent the coating from having sufficient hardness and galling resistance. Such a solid lubricant coating wears off when fastening and loosening are repeated, and it ends up completely wearing out, thereby producing metal-to-metal contact and causing galling.
In accordance with the above-described process of the present invention, drying becomes complete by performing the drying by at least two stages comprising a first stage heating at a lower temperature and a second stage heating at a higher temperature, resulting in the formation of a solid lubricant coating which has higher hardness than that obtained in the case where drying is performed by heating at a fixed temperature as employed in the prior art and which is improved with respect to galling resistance, wear resistance, adhesion, and rust preventing properties and adapted even to the environment of high-temperature oil wells.
The present invention also relates to a threaded joint for steel pipes comprising a pin and a box each having a contact surface including a threaded portion and an unthreaded metal contact portion, characterized in that the contact surface of at least one of the pin and the box has a solid lubricant coating formed thereon which comprises a lubricating powder selected from molybdenum disulfide and/or tungsten disulfide and a resin, the coating having a hardness of 70-140 on the Rockwell M scale and/or an adhesive strength of at least 500 N/m as determined by the SAICAS method.
According to another aspect, the present invention is a threaded joint for steel pipes comprising a pin and a box each having a contact surface including a threaded portion and an unthreaded metal contact portion, characterized in that the contact surface of at least one of the pin and the box has a solid lubricant coating formed thereon which comprises a lubricating powder, ultraviolet screening fine particles, and a resin binder.
A cause for a decrease in galling resistance and gas tightness encountered in a conventional threaded joint having a solid lubricant coating comprising a resin and a lubricating powder on the contact surface thereof when the joint is stored for a long period is that the rust preventing properties of the solid lubricant coating are markedly inferior to those of a compound grease so that it is not capable of completely protecting the contact surface of the threaded joint from rusting during storage. If rust is caused on the contact surface of the pin or the box during storage of such a threaded joint, the adhesion of the solid lubricant coating of the joint decreases markedly, and blistering and peeling of the coating occur. In addition, the contact surface roughness increases due to the rust. As a result, when steel pipes are connected by fastening a threaded joint, fastening becomes unstable, leading to the occurrence of galling during fastening or loosening and a decrease in gas tightness.
It was found that rusting during storage of a threaded joint having a solid lubricant coating is mainly caused by aging or deterioration with time of the resin used as a binder in the solid lubricant coating, particularly by forming cracks in the coating due to deterioration of the resin by ultraviolet light and allowing moisture to penetrate through the cracks. In order to prevent a solid lubricant coating from deteriorating by ultraviolet light, it has been found that addition of inorganic ultraviolet screening fine particles, not an organic ultraviolet absorbing agent, is effective and that rusting of a threaded joint during long-term storage is significantly suppressed by a solid lubricant coating containing ultraviolet screening fine particles.
Preferably, the ultraviolet screening fine particles are fine particles of one or more substances selected from titanium oxide, zinc oxide, and iron oxide, and they have a mean particle diameter of 0.01-0.1 xcexcm and are present in the solid lubricant coating with a mass ratio of 0.1-50 parts to 100 parts of the resin binder.
In the present invention, a lubricating powder is preferably a powder of one or more substances selected from molybdenum disulfide, tungsten disulfide, graphite, boron nitride, and polytetrafluoroethylene.
It is also preferred that the contact surface on which a solid lubricant coating is formed have a porous coating layer as a primary coat underlying the solid lubricant coating.